Signal amplifying system



Sept. 22, 1942. A. D. BLUMLEIN SIGNAL AMPLIFYING SYSTEM Filed March 29,1939 AMPLIFIER MICROPHONE OSCILLA T/O/V SOURCE i I l I I l I I I I l l lI 1 l l 1 INVENTOR ALAN DOWE BL MLEIN BY AMPLIFIER ATTORNEY PatentedSept. 22,1942 v SIGNAL AMPLIFYING SYSTEM Alan Dower Blumlein,Dialing-London, England,

assignor to Electric 3; Musical Industries Limited, Hayes, Middlesex,England, a company oi Great Britain Application March 29, 1939, SerialNo. 264,733 In Great Britain March 30, 1938 3 Claims.

This invention relates to signal amplifying systems in which provisionis made for controlling the gain of the amplifying tubes.

In both high and low frequency amplifying systems, it is known to varythe gain of an amplifier by varying manually or automatically the steadyD. C. voltage applied to a control grid and it is now common to employvariable mu-tubes in such systems.

While suchtubes are sufiiciently satisfactory for use for automatic gaincontrol purposes in broadcast wireless receivers or amplifying systemsin which changes are not frequently made, a'difiiculty arises in systemsemploying a number of amplifying channels, the outputs from which are tobe mixed. In such amplifiers the gain control is effected by applying aD. C. bias to suitable tubes from a control potentiometer connected to asource of D. C. voltage. Now it is practically impossible to ensure thatthe characteristics of the control tubes in all amplifiers are the same,so that a setting of the potentiometer which is suitable for droppingthe gain of one amplifier by 20 db. may be quite unsatisfactory fordropping the gain of another amplifier by 20 db. This difiiculty couldbe avoided by carefully calibrating the potentiometers associated witheach amplifier but the calibration would depend on the particularamplifying tubes used in the amplifier, and any change of tubes in theamplifier would necessitate a re-calibration. Furthermore, if it isdesired to control the gain of a group of amplifierssimultaneously froma master control as by simultaneously varying the D. C. voltage appliedto all the control potentiometers, it is very difficult to ensure thatthis common change of control voltage affects all amplifiers similarly.

The object of the present invention is to provide an improved method ofeffecting gain control in amplifying systems.

' According to the present invention in a ther-- mionic tube amplifyingcircuit arrangement provided with means for effecting change of gain byaltering the direct current operating conditions of one or more tubes bymeans of a direct current control, an auxiliary signal of a frequencyoutside the range of main signal frequencies to be amplified is injectedbefore the controlled tube or tubes, said auxiliary signal aftertransmission through saidtube or tubes being rectified and providingsaid direct current control, the arrangement being such that the gain ofthe amplifier, is increased or decreased when th amplicreasedrespectively. The direct current control tends to produce a constantoutput to the auxiliary signal rectifier, so that the gain of theamplifiervaries'substantially inversely to the amplitude of theauxiliary signal injected into it, substantially independently of theexact characteristics of the control tube.

The main signal frequencies may be within the audible range andconveniently in such a case the auxiliary signals are of supersonicfrequency. The amplitude of the injected auxiliary signals may becontrolled by a manually adjusted attenuator. Further, a plurality ofsuch attenuators may control the outputs from a number of amplifiersfeeding a common load, the plurality of attenuators being fed from acommon source of auxiliary signals. A further attenuator insertedbetween the source of auxiliary signals and the plurality of attenuatorsmay serve to control the gain of all the amplifiers simultaneously. Theamplifiers may conveniently be of the push-pull type as described in thespecification of application Serial No. 150,033 filed June' 24, 1937,now Patent No. 2,185,367, Jan. 2, 1940.

In order that the invention may be more clearly understood and readilycarried into effect, a low frequency amplifying system embodying theinvention will now be described in greater detail by way of example andwith reference to the accompanying drawing in which Figure 1 shows asingle low frequency input source, amplifier contude of the auxiliarysignal is decreased or innections and gain controlling arrangements andFigure 2 shows a portion of the circuit including certain alternativesin detail.

Conveniently, an amplifier system may be considered in which it isdesired to amplify and mix the outputs of a plurality of microphones,gramophone pick-ups etc., for producing an audio frequency signal whichmay be applied to a radio transmitter for a sound broadcast. For thispurpose a plurality of amplifiers are provided, one for each source ofsound, their outputs feeding into a common load. In order that theprogram may be properly controlled, it is necessary to control therelative intensities of the sounds from the various sources, and also itis convenient to have a master control by which the total volume in thefinal output may be controlled in level without afiecting the balancebetween the various sources. This may, of course, be effected byinserting suitable attenuators directly in the microphone leads whichwould serve for adjusting the relative volume from the variousmicrophones. Similarly, an attenuator might be inserted in the commonoutput of all amplifiers to adjust the general level of the outgoingsignal. This, however, would involve bringing wires carrying the soundsignals from the microphones to the control point, back to theamplifiers again, back to the control point and so to the final output.This difiiculty might be avoided by controlling the gain of theamplifiers themselves by means of D. C. bias controls to the grids ofthe amplifying tubes, this bias control serving to alter theamplification of the tubes and so effecting the necessary control. Aspreviously indicated there are inherent disadvantages in adopting thissystem. In applying the present invention to systems of this kind the D.C. control is derived for each amplifier from a rectifier associatedwith that amplifier, the rectifier being driven from a supersonic outputobtained through a suitable filter from the output of the amplifier.

v Referring to Figure 1 of the drawing, M represents a microphone orother source of low level input. A represents an amplifier foramplifying the output of this source and passing it on to the outputleads 0, together with the outputs from a number of similar amplifiers(not shown) operating from similar sources, leads from one other of'suchamplifiers only being shown. The amplifier A may be assumed to be anyconventional sound amplifier dealing with a frequency range such as 30to 10,000 0. p. s., but for the purpose of this invention it isconstructed so as to pass also up to 20 or 25 kilocycles per second. Inseries with'the microphone M is a small resistance l which is fedthrough a 20 kilocycles per second acceptor filter 2 from the controlpotentiometers shown below the dotted dividing line L and which serve tocontrol the 20 kilocycles per second supersonic input used for controlpurposes. The purpose of the filter 2 is to remove any pick-up at audiofrequencies in the wiring connecting the potentiometers so that only the20 kilocycles per second component is passed to the microphone circuit.The

control arrangements will be referred to later in greater detail.

The output from the amplifier A is led out to be mixed with the outputsfrom the other amplifiers through a 20 kilocycles per second rejectionfilter which prevents the control potentials of the other amplifiersaffecting the operation of the particular amplifier shown. A couplingtransformer 4 tuned to select 20 kilocycles per second is connectedacross the output leads of the amplifier A, this transformer having aseries tuned primary and a parallel tuned secondary. It is arranged thatthe condenser 4c connected in series with the primary Winding of thetransformer is so small as not to load substantially the output of theamplifier for audio frequencies, This transformer acts as a filterpassing only 20 kilocycles per second and steps up the voltage beforeapplying it to the, cathode of a diode 5 which is arranged to rectify anegative voltage across the resistance/condenser lead DL. A battery 6 isshown to prevent any rectification occurring until the 20 kilocycles persecond output exceeds a certain amount, the positive pole of the batterybeing connected to the cathode of the diode. The rectified outputvoltage from the diode 5, after additional smoothing if required, isapplied as a bias voltage to the controlled tubes in the amplifier A,

the whole arrangement being such that the amplification of the amplifierA tends to adjust itself so as to give a substantially constant 20kilocycles per second output .from the trans-' former 4, this outputbeing slightly greater than the voltage of the battery 6.

The control arrangements consists of a generator I which provides the 20kilocycles per second control frequency and a master attenuator 8 fromwhich a number of balance and fader controls are driven, a balancecontrol and a fader control being provided for each amplifier, one

ance and fader controls may be tapped off as, for example, from the leadII. The generator I should preferably have a substantially constantoutput and it may consist of a circuit including an oscillatorgenerating 20 kilocycles per second, the output from which is passedthrough a limiting amplifier of the form described in the specificationof Patent No. 2,185,367 previously referred to the output being filteredto remove harmonics of 20 kilocycles per second so that the resultantoutput represents a substantially steady source of sinusoidal 20kilocycles per second energy. The arrangement of the amplifier is suchthat the amplification is reduced by increasing the 20 kilocycles persecond input and therefore the various controls shown will operate in areverse manner for audio frequency currents. The gain of the amplifiervaries inversely as the amplitude of the control signal when the gain iscalculated numerically, not in dbs., and the control signal iscalculated in volts and not in dbs. above or below a certain level.

Thus, by increasing the loss of the attenuator 8 the gain of allamplifiers in the group is increased, thereby increasing the generaloutput level. The attenuator 8 therefore operates as a master controlfor the group and may be used in conjunction with a conventional levelindicator which may be bridged across the output leads 0. The balancecontrol 9 serves to control the 20 kilocycles per second level to theamplifier A and is shown as a potentiometer having a limited range ofmovement, allowing the gain of the amplifier A relatively to otheramplifiers to be controlled apart from the setting of the fader Ill. Thefader It! consists of a variable resistance preferably of high valuecompared with the potentiometer 9 and is varied to fade-in the amplifierA. Between the fader and the amplifier is connected a small attenuatorl2 consisting of a shunt and a series arm controlled by a switch.

It can be arranged that when the fader resistance ID has been removed toits lowest value, representing a large 20 kilocycles per second inputand therefore a low audio output, the fader may operate the switchcontrolling the attenuator so as to cut out this attenuation, therebylargely increasing the 20 kilocycles per second input to the amplifierand thus efiectively making its amplification for audio frequenciesnegligible.

In order to cut-ofi the outputs from the WhOFJi? group of amplifiers, itmay be arranged when the master fader adjuster reaches the position ofminimum attenuation, it operates a switch which open circuits the earthconnection to the potentiometers 9 and attenuators l2, thereby greatlyincreasing the input to the whole group of amplifiers, so reducing theiroutputs to a negligible value. I

It may be arranged that the input is effected not by a small resistanceas shown, but from a small 20 kilocycles per second transformer similarto that shown at 4 inserted in the grid lead of the first tube of theamplifier A.

An alternative form of amplifier to which the invention may be appliedmay with advantage be of the push-pull construction described in thespecification already mentioned although unbalanced amplifiers may beused. Such a pushpull amplifier may comprise, for example, four stages,in which case the first stage consistsof a pair oi high slope triodeswhich are used to raise the microphone signal clear of tube noise. Thenext two stages comprise variable mu-tubes used for amplification andgain control'and the final stage constitutes the output stage. Negativebias is applied to the grids of the two controlled stages, it beingarranged, however, that a smaller amount of controlled negative bias isapplied to the second of these stages in order to avoid overloading withvery strong input signals. This bias is obtained from the cathode of aD. C. amplifier tube, the grid of which is driven from a small dioderectifier bridged between the grid and a point of theorder of voltsnegatial of the cathode oi the tube 22, due to the bias resistance 20,prevents the rectifier operating, unless the output from the transformersecondary exceeds the bias. This in eflect constitive with respect tothe cathode on the high.

cathode resistance which is used with this tube.

This rectifier is fed by a small tuned transformer similar to a radioreceiver I. F. transformer. This transformer which is tuned to 20kilocycles has its primary in series with the anode leads of the outputpair of tubes. The input of this amplifier is fed from the secondary ofa microphone transformer, the earthy ends oi the two halves of.which areconnected to ground through two small resistances into which 20kilocycles per second input is fed through a transformer from thecontrol potentiometers. Artificial line attenuators may be used insteadof those shown in Figure 1, or tubes may be inserted between the mainattenuator common to all amplifiers, and the auxiliary attenuator inorder to prevent interaction between the attenuators 9 or in order toallow a wide range of control without requiring a large power from thegenerator 1. These amplifiers may conveniently consist of push-pullstages connected as described in the previously mentioned specification,'so that in order to cut-ofi' all the amplifiers, the attenuator 8 isset to a very low attenuation which drives these amplifiers to aconstant limiting output.

In order to completely cut-off the gain of the amplifiers, an auxiliaryrectifier arrangement may be arranged which with inputs of largeamplitude, serves to bias ofi completely a tube in the amplifier. Suchan arrangement is shown for example in Figure 2 in which 2| and 22represent two tubes in the amplifierA. These tubes are shown as beingresistance coupled with anode resistance 23, coupling condenser 24 andgrid leak 25, the value of the anode resistances employed in theamplifier being such that the characteristic of the amplifier issubstantially fiat up to 20 kilocycles per second. The tube 22 has aconventional biasing arrangement consisting of resistance 26 andcondenser 21. In. the arrangement shown, it is assumed that the level atwhich he tubes 2| and 22 work is such that no input is to be expected ofsuch a high ampli tude level as to cause the tube 22 to operate beyondthe linear portion of the characteristic. In a particular case the tube2i may be the first tube in the amplifier. The input circuit of atetrode tube 28 is coupled to the output of the tube 2|, and the primarywinding of a 20 kc. tuned transformer 29 is connected in the anode leadof the tube 28. The secondary of this transformer operates on acondenser resistance load ll placed between earth and the end of thetutes a delay. If however a very large -20 kc. input is applie' to theamplifier it will be sufficient together with the large gain availablefrom the screened grid tube 28, operating into the tuned transformer 29,to produce a rectified voltage across the load 30 which will bias thegrid of the tube 22 beyond cut ofl, thus annulling the output of theamplifier. In other words, when the fader in of Figure 1 is turned toits minimum audio output position, (maximum 20 kilocycles per second dueto the attenuator l2 being removed), the-input to the amplifier will besufllcient to operate the rectifier 3| and cut of! the tube 22.

It' may be foundin practice that with such an arrangement trouble isexperienced on first fading in a'namplifier, due to the fact that thenormal volume control line as driven by the rectifier 5, no flon'ger hasa negative potential on it. Such a diiliculty may be avoided, for

I example, by connecting a cathode of a diode to the top of the load andconnecting theanode of the diode to the anode of the rectifier 5 inFigure l.v Thus the-negative voltage on the top of the load 30 willmaintain a negative charge on the condenser in the load DL of therectifier 5.

Other methods of cutting oif one or all ofserving to close an auxiliaryD. C. circuit which either serves to bias back the amplifier completelyor to operate a relay for disconnecting the amplifier. Alternatively, itcan be arranged within the amplifier that once the auxiliary signaloutput exceeds a high value or that once the negative bias applied tothe tubes exceeds a definite high value, a relay is brought intooperation which serves to disconnect the output of the amplifier fromthe common line fed by the other amplifiers.

It is necessary to ensure that the amplitude of the 20 kc. componentused to control the amplifier is not so large as to cause overloading,so that it is desirable that an efficient coupling'gbearranged betweenthe output stage of the amplifier and the. auxiliary signal rectifier inorder that no large output is necessary from the amplifier tube in orderto provide the necessary bias. In order to obtain a sufiicient negativevoltage without requiring too great an output of auxiliary signal theoutput from the rectifier may be applied to a D. C. amplifier which, inturn, supplies the negative voltage for the controlled tubes.

While manual adjustment of the amplitude of the auxiliary control signalhas been assumed in the arrangements described, it will be understoodthat automatic operation may be achieved. Thus, the amplitude of theauxiliary control signal may be controlled electrically for example,

'by a variable gain tube. the bias of the varia- 'i'requencies of thevoltages being amplified by it will be understood that the invention isof general application to signal amplifying systems and while aparticular auxiliary control frequency of 20 kilocycles per second hasbeen referred to as a convenient frequency for use in connection withthe systems described, it will also be understood that the controlsignal frequency may be of any desired value sufficiently removed forpractical purposes from the main signal frequencies to be amplified.

Iclaim:

1. In an amplifying system a plurality of amplifiers each comprising an.electron discharge device having input and output electrodes with meansfor impressing voltages to be, amplified on theinput electrodes of eachof said amplifiers and a common load circuit coupled with the outputelectrodes of each of said devices, means for controlling the gain ofthe individual amplifiers as an inverse function of the amplitude of acontrol voltage including, a local source of control voltages of afrequency outside the range of the frequencies of the voltages impressedon the inputs of the plurality of amplifiers, separate circuits eachparallel-tuned to the frequency of the voltages from said source coupledto the input electrodes of the device in each of said amplifiers, amaster voltage amplitude regulating means coupled to said source,individual voltage amplitude adjusting circuits and voltage fading meanscoupling said-master voltage amplitude regulating means to each of saidseparate parallei-tuned circuits, separate filter circuits connectedwith the output electrodes of the device in each of said amplifiers,said filter circuits being tuned to pass voltages of a frequency of theorder of the frequency of the voltages of said source, a gain controlcircuit connected with the input electrodes of the device in eachamplifier and separate rectifiers each having output electrodesconnected with one of said gain control circuits and input electrodescoupled to one of said filter circuits.

'2. In an amplifying system an amplifier including a plurality ofelectron discharge devices in cascade, said devices having input andoutput electrodes with means for impressing voltages to be amplified onthe input electrodes of one of said devices, a load circuit coupled withthe output electrodes of another of said devices, means for controllingthe gain of the amplifier as an inverse function of the amplitude of acontrol voltage including, a source of control voltages of a frequencyoutside the range of the the amplifier, a circuit responsive to voltagesof the frequency of the voltages from said source coupled to the inputelectrodes of said one of devices, a voltage amplitude regulating meanscoupling said iast'named circuit to said source, a voltage selectingcircuit connected with the output electrodes of a device in saidamplifier, said selecting circuit being tuned to pass voltages of afrequency of the order of the frequency of the voltages of said source,a gain control circuit connected with the input electrodes of one ofsaid devices, a rectifier having output electrodes connected with saidgain control circuit and input electrodes coupled to said last namedselecting circuit, and means for biassing one of said devices in saidamplifier to cut-ofl comprising a second rectifier having inputelectrodes coupled to the output electrodes of one of said devices to beenergized by amplified voltages of the frequency of the voltages of saidsource, said second rectifier having output electrodes, and a gaincontrol circuit connecting the output electrodes of said secondrectifier to the input electrodes of a device in said amplifierr 3. Inan amplify system, the combination of a source of signals to beamplified, an output .circuit, an electron discharge device having inputelectrodescoupled to said source of signals. and having outputelectrodes coupled with said output circuit, means for controlling thegain of said device to thereby control its ability to transfer voltagesfrom its input electrodes to its output electrodes including a localsource of alternating voltage of a frequency outside of the range offrequencies of the currents supplied by said source of signals to theinput electrodes of said device, a circuit including voltage amplitudeadjusting means and voltage frequency selecting means coupling saidsource of alternating voltage to the input electrodes of said device, animpedance connected to electrodes of said device, a rectifier havingelectrodes connected with said impedance connected with electrodes ofsaid device to control the devices gain as an inverse function of theamplitude of the alternating voltage supplied by said source to theinput electrodes of said device, and a circuit for selecting voltage ofthe frequency of the voltage of said alternating voltage source couplingthe output electrodes of said device to the electrodes of saidrectifier.

ALAN DOWER. BLUMLEIN.

